This invention relates to a process for producing a calcium silicate insulation, and more particularly to a process for producing a calcium silicate insulation consisting mainly of synthetic xonotlite crystals.
As is well known, synthetic xonotlite crystals are industrially obtained by treating silica raw materials, lime and water under saturated steam at a temperature of 180.degree. to 220.degree. C in an autoclave. At the initial stage of the reaction, quasi-crystalline materials called "C--S--H(1)" are formed, but the quasi-crystalline materials are further converted to tobermorite crystals, if alumina is contained in the raw materials, and their transition to the xonotlite crystals is very difficult to take place. An increase in yield of the xonotlite crystals greatly depends upon the reactivity and grain sizes of silica, and the size and form of the xonotlite crystals also greatly depend upon the kind and grain size of silica.
Heretofore, diatomaceous earth, slag, etc. have been widely utilized as silica raw materials. However, these raw materials contain a large amount of alumina, and have larger grain sizes and a low reactivity with lime. Therefore, it is very difficult to produce calcium silicate containing a large proportion of xonotlite crystals and having a low bulk specific gravity from these silica raw materials.
Synthetic amorphous silica can solve these problems, but is so expensive that the calcium silicate containing synthetic xonotlite crystals cannot be produced at a low cost therefrom.
Furthermore, asbestos fibers are usually mixed into the raw materials to lower the bulk specific gravity and increase the mechanical strength. However, the asbestos fibers have been found in the recent years to give an unfavorable influence upon the health of the handling workers, and it is thus requested not to use the asbestos fibers. However, even if calcium silicate having a bulk specific gravity of 0.10 to 0.20 g/cm.sup.3 is attempted to be produced without asbestos fibers, the resulting products develop cracks and cavities therein, are of less strength, and larger products cannot be obtained.